This invention relates to an electronic control fuel injection system for a spark ignition internal combustion engine and, more particularly, to a technique for electronically controlling the fuel injection system for controlling the air flow rate as a function of fuel flow rate.
From the advent of the internal combustion engine to recent times, a carburetor has generally been used to supply air and fuel to the combustion chamber of a spark ignition internal combustion engine. Although a carburetor is recognized as being a superior device for adjusting an air/fuel mixture from the viewpoint of its cost performance, it is too complicated to accurately perform some of the precise adjustments needed in supplying fuel to an automotive engine. Particularly, the carburetor alone is unsuited for satisfying the demands of both fuel economy and low exhaust emissions and it is typically assisted by a fluidic correcting device, an electronic correcting device or a combination of the two for providing various air/fuel mixture correcting functions.
As an improvement over the carburetor, the Bendix Corporation has developed and widely sold an electronic control fuel injection system (EFI) which utilizes modern electronic techniques to adjust the air fuel mixture. In this system, a carburetor is not used to manage the air fuel ratio, but rather an electronic circuit is used to develop a control signal representative of the air fuel ratio which meters fuel delivery with an electronic actuator. This system takes into consideration a variety of factors in order to satisfy requirements of environmental conditions, emission levels, load performance, and fuel economy. Even though more expensive than a conventional carburetor, this system is widely used because of its many other advantages.
However, in both a carburetor and this EFI system, the air fuel ratio of the fuel mixture supplied to the engine is controlled by an operator's depression of an accelerator pedal to open or close an intake air throttle valve attached to the engine. Both select the air flow rate by this depression, suitably detect the intake air flow rate, and determine the fuel flow rate in balance with the air flow rate. That is, the air flow rate is selected preferentially as an initial value, and the fuel flow rate is then calculated as a function of the air flow rate.
However, it has been found that a conventional air preferential system cannot obtain both fuel consumption economy and clean combustion under all operating conditions of an engine. More specifically, it is different to achieve consistent fuel economy and a desired low emission density because the operating mode of a throttle valve with respect to the transient operation of the engine and the fuel flow rate pattern determined according to the operating mode of the throttle valve, as well as the time history of the air fuel ratio (A/F) at any given instant, all affect fuel economy and emission density and the driving performance of an automotive vehicle, and they often interfere with each other. For this reason, it is substantially difficult to achieve compatibility among these factors. Because the air flow rate, which is selected initially by the operator, is frequency varied stepwisely as desired, and since the air density is much lower than fuel density, a carburetor can more quickly change the air flow rate than the fuel flow rate so that the air called for at a selected air fuel ratio reaches the engine before the fuel change associated with the selected air fuel ratio. Further, in an accelerating state of the engine, the differential pressure between the front side and the rear side of the throttle valve operating as an intake air control valve becomes large up to the time when it is stepwisely varied, so that a great deal of air flows into the throttle valve at the initial time of stepwise change of the valve. Both situations result in a lean air fuel mixture. Accordingly, it is necessary to correct an excessively lean air fuel mixture ratio by adding a great deal of fuel to maintain the air fuel mixture in the combination chamber of the engine within a desired combustible range. If the correction is insufficient, the automobile's driving performance deteriorates, while if the correction is excessive, fuel economy and emission density deteriorate. Thus, the amount added is very critical.
In the case of steping down the throttle (releasing the accelerator), an opposite phenomenon occurs which has similarly critical characteristics.
Because of above problems, the air flow rate preference which has been widely adopted is of doubious value, and it is accordingly now considered better to have a fuel preference system. A good comparison between the two different systems is disclosed in Paper No. 780346 of the Society of Automotive Engineers by D. L. Stivender entitled "Engine Air Control--Basis of a Vehicular Systems Control Hierarchy."
A basic fuel preference system was initially disclosed in a U.S. Pat. No. 3,771,504 entitled a "Fluidic Fuel Injection Device Having Air Modulation", and reported in Paper No. 78-WA/DSC-21 of the American Society of Mechanical Engineers (ASME) entitled "An Air Modulated Fluidic Fuel Injection System" with respect to actual experiments conducted on the system. The fundamental concept disclosed in this patent and the report is to control the air fuel ratio as a function of the fuel flow rate in the fuel preference system by carrying out the detection, computation and actuation of the system by a pneumatic and/or fluidic circuit. This system has a good cost performance when compared with a conventional carburetor.
While this system significantly improves control over the air fuel ratio, particular during transient engine operations, since the system is essentially carried out with fluidic control, its response is somewhat slow to changing operator input, and the operating range over which adjustments in the air flow and fuel flow rate can be obtained is somewhat limited. This in turn limits the ability of the system to properly operate under all possible operating states of an engine. Also the system cannot compensate or "fine tune" the selected fuel flow rate or air flow rate to finely adjust the air fuel ratio in accordance with compensation factors determined by engine operating conditions, and cannot satisfactorily accommodate the often conflicting requirements of fuel economy and low emissions.